A scanning probe microscope (SPM) operates by scanning a surface of a sample with a probe, wherein the probe generally includes a probe body, a cantilever connected to the probe body and a probe tip attached to one end of the cantilever.
FIG. 1 shows an example operation of an SPM. As illustrated in FIG. 1, if a probe tip 110 attached to a cantilever 120 scans a surface of a sample 130, an interaction between the probe tip 110 and the sample 130 is detected and, further, the detected result is converted into an image.
Specifically, a laser beam generated from a light source 150 is irradiated on an upper surface of the cantilever 120. Then, if the probe tip 110 vertically moves along a bent of the surface of the sample 130, a reflection angle between a laser beam irradiated on the upper surface of the cantilever 120 and a laser beam reflected from the surface of the sample 130 becomes changed.
Accordingly, a position-sensitive detector 140 located in a moving direction of the reflected laser beam detects a movement of the reflected laser beam and, then, the detected result is converted into an image.
A performance of the above-described SPM depends greatly on characteristics of the probe tip 110. The characteristics of the probe tip 110 can be evaluated in terms of a height, an apex radius of the probe tip, an aspect ratio of the probe tip or the like.
Especially, the aspect ratio of the probe tip is a critical feature to determine a detection resolution of the SPM.
FIG. 2A shows an example of a probe tip 110 having a small aspect ratio, and FIG. 2B provides an example of a probe tip 110 having a large aspect ratio.
As shown in FIG. 2A, if a right-angled bent of the surface of the sample 130 is scanned by the probe tip 110 having the small aspect ratio and, then, the result thereof is converted into an image, the bent of the surface of the sample 130 cannot be precisely visualized.
On the other hand, as illustrated in FIG. 2B, if a right-angled bent of the surface of the sample 130 is scanned by the probe tip 110 having the large aspect ratio and, then, the result thereof is converted into an image, the bent of the surface of the sample 130 can be much more precisely visualized.
That is, a larger aspect ratio of the probe tip 110 enables a more precise visualization of the bent of the surface of the sample 130.
Meanwhile, as disclosed in U.S. Pat. No. 5,021,364, most of the conventional probe tips have been fabricated by a wet etching process or an isotropic dry etching process on a {100} crystal face of a {100} single-crystalline silicon. However, if such processes are employed, it is difficult to obtain a silicon probe tip of a large aspect ratio at a crystallographic point.
FIG. 3A offers a drawing of a probe tip fabricated by using the {100} single-crystalline silicon disclosed in U.S. Pat. No. 5,021,364. As shown in FIG. 3A, if the probe tip is fabricated by using the {100} single-crystalline silicon, an angle of just 54.7° is crystallographically formed by the {100} surface of the cantilever and a {111} surface of the probe tip.
Meanwhile, FIG. 3B shows a probe tip fabricated by using a {111} single-crystalline silicon in accordance with an embodiment of the present invention. Referring to FIG. 3B, if the probe tip is fabricated by using the {111} single-crystalline silicon, an angle of 70.5° is crystallographically formed by a {111} surface of the cantilever and a {111} surface of the probe tip. Accordingly, it is possible that the probe tip having a relatively larger aspect ratio is fabricated by using the {111} single-crystalline silicon in comparison with using the {100} single-crystalline silicon. Detailed description will be followed with reference to FIGS. 3C and 3D.
FIG. 3C is a diagram showing a crystal orientation of a silicon wafer; and FIG. 3D illustrates a cross sectional view of a crystal face taken along a line A-B of FIG. 3C. As shown in FIGS. 3C and 3D, if an orientation of a flat zone of a silicon wafer is set to be a [1-10] direction, an angle formed by another {111} silicon crystal face and a wafer surface becomes 19.5°. In accordance with the embodiments of the present invention, a probe tip having a large aspect ratio can be fabricated by forming the above-described {111} surface as an inclined surface of the probe tip.
Further, as disclosed in U.S. Pat. No. 5,021,364, a method for fabricating a cantilever by using the {100} silicon wafer includes a step of forming a P-N junction after doping impurity on the silicon wafer and a step of performing an etch stop process after carrying out a wet etching process on the silicon wafer with a electrochemical manner.
Moreover, another method for fabricating a cantilever by using the {100} silicon wafer is a method of using a silicon on insulator (SOI) wafer, as disclosed in U.S. Pat. No. 5,811,017. The SOI wafer is formed by bonding two wafers with an insulator film, and the SOI wafer has been widely used because the insulator functions as an etch stop layer in fabricating the cantilever.
Since the SOI wafer is much more expensive than a general silicon wafer ten times or more, manufacturing cost is increased if a probe is fabricated by using the SOI wafer.
Furthermore, if the SOI wafer is used, a thickness of the cantilever of the probe is determined by a thickness of a device layer of the SOI wafer, wherein the device layer is a silicon layer positioned on the insulator film. Accordingly, a manufactured condition of the SOI wafer can effect on the thickness of the cantilever.
In other words, the thickness of the device layer of the SOI wafer has an error during manufacturing courses of the SOI wafer. Accordingly, if there is an error in the thickness of the device layer, the thickness of the cantilever of the probe has an error, too.
After all, it is required to fabricate a probe for use in a scanning probe microscope while using inexpensive general silicon wafer, wherein the probe has a high dimensional accuracy and a crystallographically large aspect ratio.